http://citycyclingedinburgh.info/bbpress/ CityCyclingEdinburgh Forum » Topic: 20 mph in South Edin - call Kaye tomorrow? en Fri, 22 May 2026 02:46:15 +0000 http://citycyclingedinburgh.info/bbpress/topic.php?id=9231&page=2#post-109476 Thu, 02 May 2013 20:23:45 +0000 neddie 109476@http://citycyclingedinburgh.info/bbpress/ <p>20mph - Not only in South Edinburgh, but EU wide...</p> <p>Please sign the EU wide petition for 20mph as a default limit here: <a href="http://www.20mph4.eu" rel="nofollow">http://www.20mph4.eu</a> </p> http://citycyclingedinburgh.info/bbpress/topic.php?id=9231&page=2#post-96209 Wed, 09 Jan 2013 01:35:58 +0000 Uberuce 96209@http://citycyclingedinburgh.info/bbpress/ <p><em>Jaw, meet ground moment.</em></p> <p>One more before bedtime. If you were accelerated to the same speed as the protons in the Large Hadron Collider were this year, you'd be in the region of three thousand times more massive than you are now.<br /> Next year they plan on doubling that. </p> http://citycyclingedinburgh.info/bbpress/topic.php?id=9231&page=2#post-96207 Wed, 09 Jan 2013 01:14:18 +0000 Tulyar 96207@http://citycyclingedinburgh.info/bbpress/ <p>@edd1e_h made a serious assumption that it was a mile of running - sprinters probably peak at slightly more than 20mph but in running down lunch I reckon that my forebears were clocking close on that speed for sustained running (ie a bit more than an explosive sprint)</p> <p>Personally I'm rather chuffed to hear that the standard Kg seems to have shrunk..... </p> http://citycyclingedinburgh.info/bbpress/topic.php?id=9231&page=2#post-96206 Wed, 09 Jan 2013 01:09:42 +0000 Cyclingmollie 96206@http://citycyclingedinburgh.info/bbpress/ <p>"Oh, it's about ten kilometres/day's worth of inaccuracy."</p> <p>Jaw, meet ground moment. </p> http://citycyclingedinburgh.info/bbpress/topic.php?id=9231&page=2#post-96203 Wed, 09 Jan 2013 00:32:51 +0000 Uberuce 96203@http://citycyclingedinburgh.info/bbpress/ <p>And is 'relativistic friend called Uberuce' just a polite way of saying my mass increased over Christmas? </p> http://citycyclingedinburgh.info/bbpress/topic.php?id=9231&page=2#post-96200 Tue, 08 Jan 2013 23:54:47 +0000 Wilmington's Cow 96200@http://citycyclingedinburgh.info/bbpress/ <p>Yes, but <em>is</em> the cat in the box making a sound? </p> http://citycyclingedinburgh.info/bbpress/topic.php?id=9231&page=2#post-96195 Tue, 08 Jan 2013 23:09:52 +0000 Uberuce 96195@http://citycyclingedinburgh.info/bbpress/ <p>Ach, the light sail effect. Dur. Yep, that's the relativistic mass of the photons being re-emitted during reflection.</p> <p>Easily confused with solar wind, due to the sail name. Solar wind is various types of charged particle flying from the Sun. They've got plain old clunky mass, so it's a bigger push than the momentum from the light. </p> http://citycyclingedinburgh.info/bbpress/topic.php?id=9231&page=2#post-96194 Tue, 08 Jan 2013 23:05:52 +0000 Darkerside 96194@http://citycyclingedinburgh.info/bbpress/ <p>Slight aside (if that's even possible, given where this thread started...) - there was a really neat example of how time runs slow if you go fast, that finally made it click in my mind.</p> <p>Say you have a plane that's 2 things long, flying through the air at 2.5 things per second. I want to fire a beam of light from the back of the plane to the front, and light travels at 10 things per second. Speed = distance/time. So time = distance/speed. Groovy.</p> <p>I'm sat on the plane. My light gun fires from the back of the plane and travels 2 things at 10 things per second until it hits the nose. This takes 2/10 seconds, so 0.2s.</p> <p>My handy relativistic friend called Uberuce is sat on the ground watching me fly past. The light gun fires, and the beam heads towards the nose at 10 things per sec. But Uberuce can see that the plane is also moving - the nose is running away from the beam of light. After 0.2 seconds the beam has again travelled 2 things, but the nose isn't there. The plane is travelling at 2.5 things per second, so in 0.2 seconds the nose has gone 0.2x2.5=0.5 things. The light can't go any faster - its already running at the universal speed limit. It simply has to travel for some more time before it reaches the front.</p> <p>Therefore exactly the same thing takes 2 seconds for me in my speeding plane, and 2-and-a-bit for the stationary Uberuce. My time is running slow, simply because I'm moving.</p> <p>The great thing is, this isn't just trickery with numbers - if yo undo this experiment you can measure the difference, and as mentioned above, GPS devices have to take it into account all the time. </p> http://citycyclingedinburgh.info/bbpress/topic.php?id=9231&page=2#post-96190 Tue, 08 Jan 2013 22:33:55 +0000 Dave 96190@http://citycyclingedinburgh.info/bbpress/ <p>What's the mechanism that makes the little windmill of gold leaf in a vacuum spin around? </p> <p>Something to do with light, evidently. Mass? Or is it a tricksome thing to do with heat? </p> http://citycyclingedinburgh.info/bbpress/topic.php?id=9231&page=2#post-96188 Tue, 08 Jan 2013 22:19:32 +0000 Uberuce 96188@http://citycyclingedinburgh.info/bbpress/ <p>Photons have zero intrinsic mass, but due to e=mc^2, they do have relativistic mass. In this case the equation's rejigged to m=e/(c^2) so it's a hilariously tiny amount of mass, but it is non-zero.</p> <p>I think that doesn't do anything except cause gravitation, and I can't see how it can do anything else since it still has no intrinsic mass but it's been over a dozen years since I did any physics. </p> http://citycyclingedinburgh.info/bbpress/topic.php?id=9231&page=2#post-96181 Tue, 08 Jan 2013 21:43:16 +0000 DaveC 96181@http://citycyclingedinburgh.info/bbpress/ <p>Darn, Arellcat beat me to it... </p> http://citycyclingedinburgh.info/bbpress/topic.php?id=9231&page=2#post-96180 Tue, 08 Jan 2013 21:25:26 +0000 Arellcat 96180@http://citycyclingedinburgh.info/bbpress/ <p>I think Tulyar's stride length would probably make that possible. :&gt;</p> <p>Do photons have zero mass? If light behaves both as a wave and as particles, and since light travels at the speed of light, must it also have zero kinetic energy? Or is it the propagation of 'light' that travels at the speed of light, rather than its constituent parts? </p> http://citycyclingedinburgh.info/bbpress/topic.php?id=9231&page=2#post-96175 Tue, 08 Jan 2013 21:10:21 +0000 neddie 96175@http://citycyclingedinburgh.info/bbpress/ <p>If you ran at 20mph, you would cover 1 mile in 3 minutes. Tuylar just invented the 3 minute mile ;) </p> http://citycyclingedinburgh.info/bbpress/topic.php?id=9231&page=2#post-96174 Tue, 08 Jan 2013 21:05:06 +0000 Darkerside 96174@http://citycyclingedinburgh.info/bbpress/ <p>So many exciting things! Really accurate clocks, really weird science, really complicated maths that even basic phones can work out now, the Galileo GPS replacement.</p> <p>And the fact that the US could turn off GPS with a switch. Would make for a bizarre few days... </p> http://citycyclingedinburgh.info/bbpress/topic.php?id=9231&page=2#post-96171 Tue, 08 Jan 2013 20:29:33 +0000 Uberuce 96171@http://citycyclingedinburgh.info/bbpress/ <p>Trivia time: your GPS is an ongoing experiment that verifies the General and Special Theories of Relativity.</p> <p>GPS satellites are constantly ticking, each tick being a time and position signal that has eight or nine decimal places of accuracy thanks to the atomic clocks they carry and the efforts of the US military in tracking all their satellites to within a few metres. </p> <p>This allows anyone that hears a tick to know exactly how far away they are from the clock. If you don't know anything else except that distance, then all you know is that you're somewhere on a sphere which has that distance for its radius, and that satellite for its centre.</p> <p>That's obviously not much help in determining where you are, since most of that sphere is in space and some of it will be under the Earth's crust. Even if your GPS ignores all the impossible points of the sphere*, it describes two honking great arcs across the surface. The exception to this is when the satellite is directly overhead, but that's like a stopped clock telling the right time twice a day.</p> <p>So, you need more than one sphere, ie more than one satellite, to work out where you are. The US military has now got 32 of them up there now, meaning at least 4 are in direct line of sight no matter where you are on Earth at any time.</p> <p>Your GPS device is a computer that works out the solution to the problem: <em>where do I have to stand to be at the intersection of these spheres</em>? That's always a unique solution, and due to the 8/9 decimal places of accuracy, it's an sphere 5-10 metres across. </p> <p>The reason I've told you all this is because you've already heard in this thread and from other places that Relativistic effects are far too tiny to be noticed in normal daily life, and that's why we still teach people the incomplete Newtonian equations. Well, GPS is the one normal daily thing that works to such a high degree of accuracy that the difference shows up.</p> <p>The velocity of the satellites means their time signals would rapidly come out of sync. That's Special Relativity - they're moving fast relative to us, so time moves more slowly for them.</p> <p>The other effect is that of General Relativity. They're higher up Earth's gravity well and so time moves more quickly for them. The effect of General is larger than that of Special, so overall, we're running slow. Or they're running fast. It really is the same thing.</p> <p>Oh, it's about ten kilometres/day's worth of inaccuracy.</p> <p>*which it can't, unless it already knew the exact and entire topography of the Earth to a 5-10 metre accuracy, and if you had a device with that information stored in it, you'd no longer need GPS anyway. </p> http://citycyclingedinburgh.info/bbpress/topic.php?id=9231#post-96125 Tue, 08 Jan 2013 17:07:02 +0000 Tulyar 96125@http://citycyclingedinburgh.info/bbpress/ <p>Shows how long it was since you read the Highway Code - it includes a chart of stopping and thinking distances, and as already highlighted the dissipation of kinetic energy delivers a non linear result so the reason that you stop from 20 mph in the distance that you only slow to 26 mph from 30 mph is that you have dissipated the same amount of KE but have a load more to dissipate.</p> <p>Kaye should get Ben Hamilton Baillie to bring in some of the research.</p> <p>1) a couple of million years of extensive development (breeding) has delivered a human body which has a maximum operating speed of around 20mph (hard steady running) and can happily smack in to rocks and trees at this speed, and for a healthy body no bones will break and minimal internal damage will result (skull is IIRC at 30% of fracture capacity for a 20mph simple flat impact)</p> <p>2) this also relates to perceptive ability - above 22mph the ability to scan effectively the 120 degrees of forward vision diminishes and we require road markings and guidance systems as speeds increase. In experiments where all road signs and markings are removed the default speed drops to c.18mph as people negotiate their priority of passage.</p> <p>3) at 20mph the dynamic envelope of all vehicles is 'tighter' permitting for many roads, the addition of extra road 'lanes' or tighter spaces to roads to operate, releasing valuable urban land for more remunerative uses (ie development)</p> <p>Noise is lower, damage is lower, especially from larger vehicles, IIRC this is linked through a factor of 4 times the speed, so 1 speeding truck does as much damage as 100,000 speeding cars, and probably around 1bn cyclists.</p> <p>Pity I'm busy tomorrow down South </p> http://citycyclingedinburgh.info/bbpress/topic.php?id=9231#post-96121 Tue, 08 Jan 2013 16:47:23 +0000 steveo 96121@http://citycyclingedinburgh.info/bbpress/ <p>Its all relative... </p> http://citycyclingedinburgh.info/bbpress/topic.php?id=9231#post-96120 Tue, 08 Jan 2013 16:45:06 +0000 chdot 96120@http://citycyclingedinburgh.info/bbpress/ <p>"20 mph to special relativity in 10 posts!"</p> <p>That's slooow. </p> http://citycyclingedinburgh.info/bbpress/topic.php?id=9231#post-96119 Tue, 08 Jan 2013 16:44:10 +0000 chdot 96119@http://citycyclingedinburgh.info/bbpress/ <p>"<br /> Sally Hinchcliffe (@sallyhinch)<br /> 08/01/2013 15:39<br /> I think that only on @CyclingEdin does a discussion on 20mph speed limits quickly become a seminar on relativity</p> <p>"</p> <p>True. </p> http://citycyclingedinburgh.info/bbpress/topic.php?id=9231#post-96116 Tue, 08 Jan 2013 16:26:58 +0000 Wilmington's Cow 96116@http://citycyclingedinburgh.info/bbpress/ <p>From the end of the docum ent linked to by cb: "<em>Claims that 20mph is expensive to enforce are misleading. The police are required to enforce the speed limit whatever it is – this is no different with 20mph.</em>"... Unless you're in Edinburgh of course. </p> http://citycyclingedinburgh.info/bbpress/topic.php?id=9231#post-96114 Tue, 08 Jan 2013 16:17:31 +0000 Cyclingmollie 96114@http://citycyclingedinburgh.info/bbpress/ <p>That would really be funny. </p> http://citycyclingedinburgh.info/bbpress/topic.php?id=9231#post-96113 Tue, 08 Jan 2013 16:16:54 +0000 cb 96113@http://citycyclingedinburgh.info/bbpress/ <p>Oops, I think Aken's last post slipped into a wormhole. </p> http://citycyclingedinburgh.info/bbpress/topic.php?id=9231#post-96111 Tue, 08 Jan 2013 16:15:05 +0000 AKen 96111@http://citycyclingedinburgh.info/bbpress/ <br /> http://citycyclingedinburgh.info/bbpress/topic.php?id=9231#post-96110 Tue, 08 Jan 2013 16:15:03 +0000 Wilmington's Cow 96110@http://citycyclingedinburgh.info/bbpress/ <p>It would be <em>such</em> a funny show hijack for it to become all about Einstein and whether he was right... </p> http://citycyclingedinburgh.info/bbpress/topic.php?id=9231#post-96109 Tue, 08 Jan 2013 16:14:52 +0000 steveo 96109@http://citycyclingedinburgh.info/bbpress/ <p>Got to love this place, 20 mph to special relativity in 10 posts! </p> http://citycyclingedinburgh.info/bbpress/topic.php?id=9231#post-96107 Tue, 08 Jan 2013 16:11:00 +0000 AKen 96107@http://citycyclingedinburgh.info/bbpress/ <p>I think you should go on Call Kaye, Darkerside. If anyone's in any doubt about 20mph zones then your concise but thorough discourse on special relativity will win them over. </p> http://citycyclingedinburgh.info/bbpress/topic.php?id=9231#post-96101 Tue, 08 Jan 2013 15:59:01 +0000 Darkerside 96101@http://citycyclingedinburgh.info/bbpress/ <p>Having quickly checked my facts, exciting maths starts:</p> <p>Near the speed of light ('c') the KE equation above doesn't work, you need to use special relativity alternatives. This is because as you get close to c, time and distance start to do peculiar things, like slowing down. The energy of something of mass m travelling quickly at speed 'v' includes something called the Lorentz factor. If we cancel it out (and use sqrt for square root, we get:</p> <p>E = (mc^2)/sqrt(1-v^2/c^2)</p> <p>When something is stationary, this gives us E=mc^2. If something with mass travelled at light speed though, we'd get:</p> <p>E = (mc^2)/sqrt(1-1) = (mc^2)/0 = inf</p> <p>Infitite energy would be unpopular, therefore anything with mass can't go at light speed.</p> <p>Exciting maths ends </p> http://citycyclingedinburgh.info/bbpress/topic.php?id=9231#post-96100 Tue, 08 Jan 2013 15:51:07 +0000 Cyclingmollie 96100@http://citycyclingedinburgh.info/bbpress/ <p>Thanks for the explanation and the links to the graphs. I would have called those curves concave, like the climb up to Redstone Rig. </p> http://citycyclingedinburgh.info/bbpress/topic.php?id=9231#post-96098 Tue, 08 Jan 2013 15:49:30 +0000 steveo 96098@http://citycyclingedinburgh.info/bbpress/ <p><em>Newtonian laws of motion (from which the KE one above is derived) don't hold.</em></p> <p>Though I'd say they're quite sufficient at the speed I travel :D</p> <p><em>And does that account for the square in e=mc²?(Sorry to digress but I find this quite intesting). </em></p> <p>I <em>think </em>so but I'm no physicist. C is the speed of light. </p> http://citycyclingedinburgh.info/bbpress/topic.php?id=9231#post-96093 Tue, 08 Jan 2013 15:42:35 +0000 Darkerside 96093@http://citycyclingedinburgh.info/bbpress/ <p>And not quite on the exponential growth bit - it would display quadratic growth (ie something to the power two), but not exponential. Both are 'convex' functions (ie increasing by amounts that go up each step), but exponential is more dramatic.</p> <p>See: <a href="http://en.wikipedia.org/wiki/File:Exponential.svg" rel="nofollow">http://en.wikipedia.org/wiki/File:Exponential.svg</a>, and pretend that the scale from 0-10 is velocity (ignore the vertical scale).</p> <p>Kinetic energy would be a line below the blue one, but of similar-ish shape. If it increased, exponentially with speed, it would be more like the green one. </p>